Biology Reference
In-Depth Information
3. METAMORPHIC ASYMMETRY OF THE BRAIN,
SENSORY ORGANS, AND BEHAVIOR
In contrast with the eyes, the flatfish olfactory organs do not change
location but do display dramatic morphological asymmetry, with the eyed-
side olfactory organ, nerve, and telencephalon being significantly larger
compared with that on the blind side (
Brinon et al., 1993; Prasada Rao &
Finger, 1984
). The central olfactory projections have also been shown to
be asymmetric in the winter flounder (
Pseudopleuronectes americanus
), with
the blind side telencephalon receiving approximately equal olfactory input
from the two sides, whereas the eyed-side telencephalon is measurably larger
than and receives significantly more input from the eyed-side olfactory or-
gan (
Prasada Rao & Finger, 1984
). The development of metamorphic tel-
encephalic asymmetry in the Senegalese sole (
Solea senegalensis
) does not
appear to result from asymmetric cell proliferation within the telencephalon
(
Pinuela, Rendon, Gonzalez de Canales, & Sarasquete, 2004
). Interestingly,
using
S. senegalensis
,
Velez, Hubbard, Barata, and Canario (2005)
demon-
strated that these morphological olfactory asymmetries are accompanied
by functional asymmetry in responsiveness to different odorants by each
olfactory organ as measured by electro-olfactogram, the first report of an
asymmetric olfactory response in a vertebrate.
Despite the dramatic migration of one eye to the opposite side of the head
during flatfish metamorphosis, a documented length asymmetry between the
right and left optic nerves (
Murray, 1974
), and an unusually nonrandom con-
figuration of the dorsal-ventral crossing of the optic nerves at the optic chi-
asma in some flatfish species (the nerve of the migrating eye typically lies dorsal
to the nonmigrating eye's optic nerve) (
Ballard, Pickett, & Sivak, 1987;
Parker, 1903; Policansky, 1982b
), no evidence of asymmetric central organi-
zation of the retinal projection into the brain has yet been discovered
(
Luckenbill-Edds & Sharma, 1977; Medina, Reperant, Ward, Rio, &
Lemire, 1993
).
Brinon et al. (1993)
have, however, reported the existence
of a transient volumetric asymmetry between the left and right sides of the
optic tectum during turbot (
Scophthalmus maximus
) metamorphosis, with sym-
metry reestablished after metamorphosis. No morphological asymmetry has
ever been reported to exist in the flatfish midbrain or hindbrain. Together
with the previously described changes in metamorphic epithalamus morphol-
ogy, these findings suggest that postmetamorphic flatfish morphological asym-
metry of the neurocranium is confined to regions rostral to the optic tectum.
